This invention relates to a coke calcining apparatus suitable for producing high-grade coke, especially such high grade coke as is suitable for use in fabricating graphite electrodes, by two-stage calcination with intermediate cooling.
Preparation of green coke from heavy oils of petroleum origin such as residue oils of catalytic cracking and thermal cracking, straight run residue oils and tar of thermal cracking, coal tar pitch or mixtures thereof by a delayed coking process is known in the art. The green coke produced by this process still contains a significant quantity of moisture and volatile matter. Accordingly, there is also known a process for calcining the produced green coke in order to remove the moisture and volatile matter from the green coke and to densify the same, thereby producing a carbon material having a high density and a low coefficient of thermal expansion which is suitable for use as an electrode material for steel-making, aluminum smelting or the like or a carbon material for other shaped articles.
Calcining of such green coke is carried out in heating furnaces such as a rotary kiln, a rotary hearth, and a shaft kiln in a single stage, or in two stages by further providing a preheating furnace.
However, as a result of my studies, I have found that calcined coke obtained by this process does not necessarily have fully satisfactory properties as coke for artificial graphite electrodes which is required to be of particularly high quality. That is, there remains much room for improvement with respect to high density and low coefficient of thermal expansion which are the most important properties required of coke for artificial graphite electrodes.
On the other hand, our research staff has found that cooling in an intermediate stage in the calcination of coke is highly effective in reducing the coefficient of thermal expansion of the calcined coke and increasing the density, particularly the true density thereof, and has developed a process for producing high-grade coke. This process for calcining coke comprises first calcining green coke obtained by a delayed coking process at a temperature lower than an ordinary calcining temperature, cooling once the calcined coke, and thereafter calcining the coke again at a temperature in the ordinary calcining temperature range (as disclosed in U.S. Pat. No. 4,100,265, July 11, 1978). Although it is not sufficiently clear why the coefficient of thermal expansion of the calcined coke is reduced by intermediate cooling, a possible reason may be that some fine cracks are formed in the coke during the process wherein the coke, after being heated to a temperature of 600.degree. to 1,000.degree. C., is subjected to intermediate cooling and then to reheating, which cracks are considered to absorb expansion due to heating, resulting in the reduction of the overall coefficient of thermal expansion of the coke. The true density of the calcined coke is increased presumably because rapid evaporation of volatile matter and formation of a porous structure which occurs as a result thereof are suppressed by the intermediate cooling in the above specified temperature range.
Two-stage coke calcination with intermediate cooling is carried out by means of a coke calcining apparatus comprising, for example, two or more rotary kilns in series and a cooling device installed between them. An example of a coke calcining apparatus of this character is disclosed in Japanese Patent Laid-Open Publication 118995/1980 (Specification of U.S. Pat. No. 4,265,710) for my previous invention, in which an apparatus comprising a combination of three rotary kilns including a drying preheater and a cooler disposed between the rotary kilns of the last two stages is used.
However, the use of an apparatus of this character in which a plurality of rotary kilns are used, and coke at a high temperature is extracted at an intermediate point is accompanied by problems such as the following.
(a) Since there are several kilns, the apparatus becomes economically disadvantageous for reasons such as increased installation cost and the large space required.
(b) The number of control points becomes great, and controls such as combustion control becomes complicated.
(c) Processing and conveying of the high-temperature coke which has been extracted at an intermediate point is difficult and, moreover, entails danger.
(d) With the increase in the physical bulk of the entire apparatus, the thermal efficiency decreases.